Electronic equipment often employs printed circuit boards or cards. These cards typically are mounted in a chassis or housing by stacking the cards in row alignment with one another. Row alignment in the chassis is defined by slotted or spaced surfaces within or on the chassis, with each board placed in a slot or between a pair of spaced surfaces. A retainer may be provided in the chassis slot to captivate a card positioned therein. Many applications for such retainers require high performance that will captivate a printed circuit board under the most extreme shock and vibration conditions such as those encountered by spacecraft or military aircraft.
Typical printed circuit board retainers are described in U.S. Pat. Nos. 4,823,951 and 5,036,428, the teachings of which are incorporated herein by reference. Such retainers comprise a partially threaded screw or rod and a plurality of members slidably mounted in an end-to-end relationship on the rod. The members disposed on the rod have wedge-shaped end portions which are engagable with one another. The wedge-shaped end portions serve to move at least one of the members in a transverse direction relative to the rod when the members are moved towards one another along the rod. Such movement may be achieved by providing the rod with screw threads to engage and move the distal-most member when the rod is rotated. Alternatively, the movement may result from the action of a lever assembly used to draw the rod away from the distal-most member, thereby pulling that member toward its companions.
Commonly, the rod in prior art retainers is threaded at one end to engage with mating threads on a nut attached to the endmost sliding members mounted on the rod. The opposite end of the rod includes a portion that may be engaged by a tool to allow the rod to be rotated. In so doing, the members are moved toward one another as the threaded rod is rotated in the tightening direction.
The transverse direction in which the member is moved acts to engage a clamping surface of that member against an edge of the printed circuit board. The opposite edge of the board is thereby forced into contact with, and clamped against, a spaced surface fixedly connected to or integral with the chassis in which the board is to be mounted. Typically, the spaced surface is the wall of a housing that is screwed or riveted to the chassis.
In many high performance applications, the circuit board chassis comprises a sealed box which does not allow cooling air to pass over the circuit board. Rather, heat generated by the circuit board is conducted through a metal heat sink attached to the printed circuit board and then transmitted to a heat exchanger or plenum. The heat transfer path is that between the printed circuit board and the slot surface of the chassis or housing against which the board is clamped. The heat generated by the electrical components must be removed from the circuit board in order to maximize equipment performance and to minimize downtime. Accordingly, the housings in which the circuit boards are mounted are normally fabricated of a material having good heat conduction properties so as to also serve as a heat sink for dissipating the heat generated by the circuit board components.
Various attempts have been made in the prior art to facilitate the removal of heat from the circuit board to the housing. Typically, these efforts have included employing housing materials having improved heat conduction properties, increasing the contact area between the circuit board and the adjacent housing wall and employing pressure to eliminate void spaces at the interface formed between the circuit board and the housing wall, thereby improving the quality of the contact therebetween. Since at the microscopic level, solid surfaces are not absolutely flat, the contact interface between two solid surfaces consists of many small points of contact. These points of contact are affected by the surface smoothness, malleability and the degree of pressure forcing together the contacting surfaces. As the contact pressure between two solid surfaces is increased, the actual contact area is increased and thermal resistance between the surfaces is reduced. Conversely, at low contact pressures, much of the heat transfer is through void spaces located between the points of contact and thermal resistance at the interface of the surfaces remains high.
It is therefore highly desirable to maximize the cross-sectional contact area between the circuit board and the housing in order to create the least possible resistance to heat flow. One difficulty with prior art circuit board retainers is that heat conduction through the side of the circuit board contacting the retainer is limited because the surface area of the cams or sliding members which contact the circuit board is generally small. Thus, the efficiency of heat removal from the board for the prior art retainers is dependent upon heat conduction across the primary interface formed between one surface of the circuit board and the abutting housing wall.
In addition, the pressure exerted against the circuit board for a retainer having a discontinuous surface contacting the circuit board, for example, the above-referenced wedge-type retainer, is uneven. The uneven distribution of pressure across a portion of the circuit board increases the likelihood of breaking the circuit board as the retainer pressure is increased. Another problem with retainers having a discontinuous surface, is that heat conduction from the board through the retainer occurs independently at each discontinuous surface. As a result, some of the surfaces may receive a larger amount of heat from the board, depending upon whether a larger amount of heat is generated by circuit board components in the vicinity of one retainer surface compared to another retainer surface. Stated another way, the retainer is incapable of evenly distributing the heat removed from the circuit board. Retainers having a continuous surface contacting the circuit board can more efficiently remove heat from the board because the continuous surface can act as an isothermal plane to redistribute the heat removed from the board across the length of the continuous surface.
The prior art has recognized the problems of increased thermal resistance associated with the application of non-uniform pressure across the length of the circuit board. To solve this problem, several prior art retainers have provided a continuous surface area for pressing an edge of the circuit board against an adjacent housing wall. These retainers attempt to facilitate the removal of heat from the board by forcing a first surface of the circuit board against the adjacent housing wall, thereby increasing the actual contact area between the abutting surfaces.
For example, U.S. Pat. No. 4,721,155, issued to McNulty, discloses a circuit board retainer for pressing an edge of a circuit board heat sink, i.e., the heat conductive plate upon which the circuit board is mounted, against a housing slot wall. The circuit board retainer includes a mating pair of sawtoothed bars having slidingly engaging teeth. One of the bars is attached to the housing wall. The other bar is free to engage an edge of the circuit board. As the bars are forced apart, the second bar engages the edge of the circuit board heat sink, forcing the opposite side of the board against its adjacent housing wall.
U.S. Pat. No. 4,869,680, issued to Yamamoto et al., discloses a rod assembly for retaining a circuit board, the assembly including a generally cylindrical rod having a plurality of holes aligned along a first side of the rod. The holes are oriented substantially perpendicular to the longitudinal axis of the rod, each hole including a ball which is projected outwardly, i.e., beyond the first side of the rod, by a first biasing means. Rotation of the rod causes the plurality of balls to bear against one side wall of the housing slot. This, in turn, forces the opposite surface of the circuit board against the opposing wall of the slot.
The above-described retainers attempted to reduce thermal resistance at the primary interface by providing a retainer having a continuous surface cammed against one side of the circuit board to force the other side of the circuit board against its adjacent housing wall. Although the prior art recognized the advantage of distributing pressure along the length of the circuit board, the above-described constructions are not entirely satisfactory. In particular, these retainers do not provide optimum resistance to bending and/or twisting of the bar in contact with the circuit board under pressure and therefore may not uniformly distribute pressure over a portion of the circuit board.
In addition, the efficiency of the above-described retainers in removing heat from the circuit board is dependent on only two heat conduction paths to remove heat from the board to the housing, only one of which paths is free of air gaps. In a first heat conduction path, heat is removed from the board across a primary interface formed at the junction of one side of the circuit board and a housing wall. In a second heat conduction path, heat is removed from the board across a secondary interface formed at the junction of the opposite side of the board and the retainer. In both patents, there are air gaps in the heat flow path through the retainer, Finally, neither of the above-described retainers discloses a third heat conduction path including a third interface across which heat is removed from the circuit board to the housing.
A need, therefore, exists for a retainer assembly capable of uniformly pressing a portion of a circuit board against an adjacent housing wall. Preferably, the assembly should be designed and constructed to provide enhanced heat conduction paths for facilitating heat removal from the board. An improved assembly should also permit insertion of the assembly into an existing housing without substantial modification of the housing and/or retainer. This latter objective requires that, if the retainer is to have a bar for transferring pressure to the circuit board or other panel, the bar be designed to provide optimum resistance to bending and twisting for the space available.